A rearview mirror which combines the headlamp on/off control and the rearview mirror control functions is in commercial use. The only such system which is known to the inventors is manufactured by the assignee of this invention and is also the subject of a copending patent application Ser. No. 07/670,258, filed March 15, 1991, which is also assigned to the assignee of this invention. The commercial system utilizes a single microcontroller for the combined mirror and headlamp control functions but uses completely separate sensors for the mirror control and the headlamp control functions.
Mirror control circuits of the type utilized in an embodiment of this invention are generally described in U.S. Pat. No. 5,204,778 which is included herein by reference.
It was noted above that the conventional combined headlamp and mirror control did not utilize a common sensor for the headlamp and the mirror control functions. There were several reasons for this. First, it was commonly believed that satisfactory performance for the headlamp control function required the sensor to view skyward and that the sensor for the mirror control needed to view forwardly with the view in the skyward direction blocked. The conventional headlamp and running light controls have normally utilized a sensor which views upward and which covers a relatively wide viewing angle. In what follows, the term "sensor's field of view" refers to the total viewing region for which the sensor has substantial sensitivity to light rays emanating therefrom. The sensor for the headlamp control has traditionally been mounted under the dash of the vehicle so as to view skyward through a grille in the dash and through the windshield. The combined headlamp control and automatic mirror with the separate sensors referred to above incorporates a sensor which is mounted to view upward through the windshield at a position just below and in front of the button which is used to mount the mirror to the vehicle's windshield. The automatic mirrors, on the other hand, have normally used an ambient light sensor which views forwardly with some type of visor to intentionally limit the maximum elevational angle of the sensor's field of view. Two common restrictions for the mirror ambient light sensor have been to restrict the sensor's field of view to approximately that which the driver sees and to also to limit the elevational angle of the sensor's field of view so as to prevent undue response to light from street lamps which are nearly overhead. When seated in a normal position, the upper extent of a tall driver's elevational field of view through the windshield below the tinted shade band may be as small as 5 degrees from the horizontal. For typical drivers seated in a normal driving position the upper extent of their elevational field of view is less that 30 degrees from the horizontal for many vehicles. For the combined mirror and headlamp control function, it is desirable to violate the requirement to view generally the field of view which the driver sees (Here it is assumed that this is the view seen when the driver is sitting in a normal position and not when crouching and or leaning forward.) In the preferred embodiment, the elevational field of view is extended up to an elevational angle of sixty degrees above the horizontal when it is possible to do so with the available sensor mounting positions. This brings up an additional problem. The tinted shade band used across the top of many windshields blocks a high enough percentage of the light coming through it to effectively block the sensor's field of view through it. The ambient light sensors in many present day automatic rearview mirrors are mounted where their greatest elevational viewing angle is effectively limited to 30 to 45 degrees by the tinted shade band of the windshield. For the combined sensor application, several features should be used either singly or in combination to enlarge the sensor's field of view. First, the aperture in the case for the ambient light sensor'should be assigned for fairly minimal restriction on the sensor's field of view. This includes making the forward field of view laterally wide as possible and extending the elevational angle of the field of view to approximately 60 degrees above the horizontal. Second, a lens may be used to facilitate the enlargement of the field of view. Third, the sensor should be placed low on the mirror case and preferably toward the driver's side which is angled toward the windshield so that it will be as far down and as far forward as possible to maximize the elevational field of view as limited by the tinted shade band of the windshield or possibly by the top of the windshield if a deeply tinted shade band is not used. An alternative is to place the sensor very close to the windshield in the proximity of the mounting button of the mirror and to provide the view indicated above. As indicated, it is desirable to use a much wider field of view than has been the practice of the mirror ambient light sensing function and only to shade the sensor from light which comes from almost directly overhead to minimize the effect of street lamps which are nearly overhead on the mirror control function but to include an elevational viewing angle of up to sixty degrees or so above the horizontal and toward the front of the vehicle.
To use the sensor for the combined application, a field of view for the sensor which represents a best compromise for the available mounting position and for performance for the mirror and for the headlamp and running light control should be used. There are also technical advantages with the mirror control to make the sensor's total field of view as large as possible perhaps short of being sensitive to the direct light from streetlights which are almost directly overhead. It is advantageous to use a very large field of view for the headlamp and running light control function. Thus, the sensor position and aperture should be chosen to generally increase the sensor's field of view over that which has traditionally been used for the mirror control application. To accommodate the mirror control function, the sensor should be directed to view in a generally forward direction, preferably angling it slightly upward from the horizontal viewing direction to generally center the viewing aperture of the light sensor with desired field of view as just defined.
A second factor preventing the shared use of an ambient light sensor for the headlamp and running light and mirror ambient light sensing functions is the very large light range that must be sensed for the combined functions and the need to simultaneously supply a signal from the sensor to each of the control circuits. The range of ambient light levels for which actual light levels need to be read and for which the magnitude of these readings directly effect the response of the mirror typically span the range from roughly 0.1 lux to 50 lux. This is already a large dynamic range of five hundred to one. The light levels which are used to determine when to turn the headlamps on or off normally fall in the range of 100 to 1000 lux. Thus, the use of a single sensor to serve both functions satisfactorily requires a substantially larger dynamic range. This range must be provided inexpensively and performance must be satisfactory over the entire automotive temperature and humidity range. The circuit must also continue to perform properly in the presence of very high levels of radio frequency emissions such as those created by relatively powerful mobile communications or mobile ham radio transmitters whose transmitting antennas may be placed in close proximity to the control module. With these obstacles, it requires an inventive circuit configuration to obtain satisfactory circuit performance without the use of separate sensors for the mirror ambient light and the headlamp ambient light sensing functions. Analog-based mirror circuits other than the one of U.S. Pat. No. 5,204,778 have typically biased the ambient light sensor in ways that would provide a sensor signal level of well under 100 millivolts at light levels at which the headlamp Circuit switching thresholds are placed. The mirror control circuits some embodiments of U.S. Pat. No. 5,204,778 provide unusually high sensor voltages at the higher light levels at which the headlamps are turned on and off. The voltage at the ambient light sensor for the circuit of FIG. 2 of U.S. Pat. No. 5,204,778 is in the range of several tenths of a volt to a little over a volt at the threshold light levels for which the headlamps are turned on and off. This is much higher than the level available from the sensor configuration of other circuits and this discovery was the first step leading to the device of the preferred embodiment.
Several additional improvements over prior art devices are needed to provide good performance with the combined sensor configuration. It is important to turn the headlamps of a vehicle on when they are required and off when they are not required but at the same time to not cycle too frequently between on and off states so as to confuse drivers or pedestrians in the vicinity of the vehicle or to annoy occupants of the vehicle or to wear out the vehicle's headlamp switching mechanism. As will be explained later in more detail, the preferred circuit incorporates hysteresis whereby the ambient light level is first compared against one of two thresholds, the higher threshold corresponding to a light level which is generally in the range of 40 to 100 percent higher than the lower threshold. When the sensor reading indicates that the headlamps should be on, the ambient light level must rise above the higher threshold for the circuit to indicate that they should be turned off. When the sensor reading indicates that the headlamps should be off, the ambient light level must fall below the lower threshold for the circuit to indicate that they should be turned on. The feature just described is also incorporated in one form or another in prior art devices.
Light levels can be so erratic due to things such as varying cloud cover, buildings or trees blocking light, or changes in direction of travel that even the hysteresis used to create the dual thresholds just described is not enough to assure reasonably stable switching of the headlamps. An additional feature used by present day analog headlamp and running light control circuits has been to combine a time average with the hysteresis circuit to effectively add a delay to the headlamp switching function so that the headlight switching is delayed by a period generally ranging from five to fifty seconds after the sensed ambient light level has changed to indicate that the alternate state should be assumed. Such arrangements did lead to improved performance, but circuits which utilize them still tend to switch the headlamps on and off too frequently when there are frequent changes in the ambient light level as is, for example, caused by intermittent shadowing of clouds or by numerous overpasses. Furthermore, some compromise is involved in positioning an ambient light sensor to serve both the mirror and headlamp and running light function so this problem is exacerbated. Some microcontroller based devices such as the one of the copending application Ser. No. 07/670,258 have used more sophisticated timing arrangements but no prior art analog circuit based headlamp controls with timing features of the type used in the preferred circuit are known to the inventors. As will be explained in detail later, the preferred device of this invention utilizes a circuit feature which favors and thereby tends to keep the headlamps in their present state under fluctuating ambient light conditions.
Many presently used analog circuit based headlamp controls which are known to the inventors and which have a headlamp delay feature to keep the headlamps on for a predetermined period after the ignition is turned off continue to draw several milliamps all of the time even after the headlamps are turned off. Such controls are allowed only because they were designed before more stringent quiescent current consumption requirements were instituted to limit total quiescent current consumption. New designs must meet the more stringent current consumption requirements which are necessitated by the greatly increased number of electronic devices in today's vehicles. The analog based controls are potentially less expensive than digital ones, but need to either have extremely low quiescent current consumption or to de-energize themselves when both the ignition switch and the headlamps are off. When the units shut themselves down, an additional problem is encountered in that there must not be spurious switching of the headlamps either when the unit de-energizes itself or when the unit is re-energized when the ignition switch is turned on. Furthermore, when the ignition is turned on, the headlamps must not be energized unless the ambient light level indicates that they are needed and if the ambient light level does indicate that they are needed, they must be energized within a short time which should not exceed three to five seconds. When the headlamps are needed, it is preferable to delay the headlamp turn on for one to two seconds after the ignition is turned on. The driver normally turns the ignition key directly to the "start" position and holds it there until the engine starts. Under the stated conditions, since the headlamps are not normally energized with the ignition in the "start" position, the short delay prevents the headlamps from coming on to full brightness until after the vehicle is started.
Thus, one objective of the present invention is to provide a headlamp and running light control which uses an ambient light sensor which provides a signal which is used both to at least partially determine whether turn the headlamps on or off and to at least partially determine the operating sensitivity of an automatic rearview mirror.
Another objective of the present invention is to provide an analog based headlamp and running light control in which a time delay period is used to delay switching from one headlamp state to the other with control of the delay period characterized in that a substantially shorter time is required to increase the delay period by a given delay increment toward its maximum period than is required to expend the given delay increment in the direction toward switching the state of the headlamps. The increase in the delay period toward its maximum takes place when the desired headlamp condition as indicated by the sensed ambient light level agrees, or is consistent with the present state of the headlamps and the delay period is expended when the desired headlamp condition as indicated by the sensed ambient light level disagrees, or is inconsistent with the present state of the headlamps.
Another objective of the present invention is to provide an analog based headlamp control for which, under rapidly fluctuating ambient light conditions, the desired state of the headlamps as indicated by the sensed ambient light level needs to agree with the present headlamp state for only a relatively small percentage of the time to cause the headlamps to remain in that state. This small percentage is preferably five percent to twenty percent of the time but may be chosen in the range of zero percent to forty percent. The greatest advantage is gained by choosing a small percentage which is greater than zero so that frequent flashes of bright light cannot hold the headlamps off, or frequent very short periods of deep shadowing cannot hold them on.
Another objective of the present invention is to provide an analog based control which provides a headlamp exit delay feature and which de-energizes its own circuits when or after the headlamps are turned off and which then reinitializes itself when the ignition is turned on again to power up with the headlamps off but to turn them on within an acceptable short period of time when they are required. An additional optional objective is to supply the power to maintain operation of the circuit during the exit delay period and to signal the NIGHT mode or headlamp on condition using a single switching means. An additional optional objective is to use a single timing means to effect proper initialization of the headlamps when the ignition switch is turned on and to perform the timing function for the exit delay function.
An additional objective of the present invention is to perform the complex combination of timing initialization and power conservation functions of the headlamp control in an analog implementation which utilizes analog circuit technologies similar to those of the mirror control circuit of U.S. Pat. No. 5,204,778. By so doing, large portions of the circuit functions for both the mirror and the headlamp control features can ultimately be implemented in a single integrated circuit which will give a substantially more economical solution for many applications than a microcontroller based design. The array of features referred to above is not easy to implement in an economical analog device.
Another objective of an alternate embodiment of this invention is to use a common ambient light sensor for control of an automatic rearview mirror and for sensing light levels in the proper range for the headlamp and running light function whereby the ambient light level signal is changed to a digital indication and transmitted over a communication bus to a microcontroller based unit which uses the transmitted ambient light indication to at least partially determine the state to which to switch the headlamps.
A further objective of an alternate embodiment of the present invention is to share use of an ambient light sensor at least between the mirror and the headlamp control functions and to incorporate a microcontroller for a portion of the control function.
A further objective of the present invention is to provide a separate ambient light sensor and circuit module which may be mounted in the vicinity of or integrally with the rearview mirror and which senses the ambient light and generates a control signal to turn the headlamps on and off as a function of the signal from the ambient light sensor, the module serving as an alternate to the combined headlamp and mirror control module and characterized in that it may be used interchangeably with the mirror and headlamp control module so that different wiring or different headlamp switching modules are not required for the separate mirror options. It is anticipated that this module would normally be used in combination with a conventional prism type rearview mirror when the automatic mirror is not provided.
A further objective of the present invention is to provide an ambient light sensor for the combined headlamp and running light and mirror control functions which has a field of view to the front of the vehicle which is substantially larger than that seen by the driver.
Another objective of the present invention is to provide a sensor and associated control circuits for which the mirror control function primarily utilizes the ambient sensor circuit's current versus light level characteristic to perform its control function and the headlamp and running light control primarily utilizes the ambient light sensors circuit's voltages versus light level characteristic to perform its control function.